The present invention relates to screen printing machines in general, and more particularly to improvements in flat screen printing machines of the type wherein the screen cooperates with a counterpressure member and the path for the material which is to be provided with printing medium extends between the screen and the counterpressure member.
Screen printing machines of the above outlined character are disclosed, for example, in U.S. Pat. No. 3,120,180. In certain presently known flat screen printing machines, the screen is reciprocable in the upper portion of the machine frame and a stationary cylindrical counterpressure member is rotatably mounted in the central portion of the frame at a level below the screen. The counterpressure member is arranged to pull the material into the machine. A drawback of such machines is that they occupy an inordinately large amount of space because the entire screen must be moved to both sides of the counterpressure member. As a rule, the just described machines are used for the application of printing media to workpieces (e.g., paper sheets) of finite length.
In the so-called flat bed printing machines, the screen is placed on top of the workpiece. A squeegee is thereupon caused to move back and forth within the confines of the screen and to force the printing medium through the interstices of the screen and into contact with the workpiece. The screen is thereupon lifted, the workpiece is shifted relative to the frame so as to locate an untreated portion at the printing station, and the cycle is repeated. A drawback of such machines is that the screen must be lifted off the workpiece together with a pool of printing medium thereon which creates problems during actual separation from the workpiece as well as during renewed movement of the screen into surface-to-surface contact with an unprinted portion of the workpiece. This often results in smudging of the screen and/or workpiece and can affect the appearance of the pattern which is applied to the workpiece. In many respects, the aforediscussed machines with reciprocable screens operate more satisfactorily than a flat bed printing machine wherein the screen is stationary in the course of the printing operation. Thus, the application of printing medium is more predictable and the workpiece as well as the screen are less likely to be contaminated by printing medium in regions which should not be contacted by such medium or (in the case of the workpiece) are already coated with an ink, a dye, a lustre enhancing medium or the like.